The long-term objectives of this study are to characterize the gene defects leading to familial hypophosphatemic rickets (FHR), to understand how phosphate transport is regulated, and to lay the ground work for better therapy for affected individuals. PEX is the gene responsible for the most common form of FHR, X-linked hypophosphatemic rickets (HYP). In previous studies, no PEX mutation was detectable in about 50% of individuals with FHR, suggesting that some affected individuals may have autosomal dominant hypophosphatemic rickets (ADHR), another form of FHR for which the genetic defect is not known. The hypothesis is that mutations in genes other than PEX may lead to the HYP phenotype, and that these genes play an important role in phosphate transport. Specific Aims: 1) Analyze FHR patients for mutations in PEX 2) Perform a genotype-phenotype analysis of PEX mutations in patients with HYP 3) Identify a subset of patients with FHR and no detectable mutation in PEX, who will be used to search for other loci responsible for ADHR, with special emphasis on chromosomes 9 and 13.